Auto-servo tape system and associated recording head

ABSTRACT

A recording head comprises a first tape-head contact area including a first write element and a first read element, which itself may include an MR sensor element. The first read element is laterally offset as to the first write element, and no portion of the first read element lies in a region laterally overlapped by the first write element. A second tape-head contact area includes a second write element and a second read element, which also may include an MR sensor element. The second read element is laterally offset as to the second write element, and no portion of the second read element lies in a region laterally overlapped by the second write element. The first read and write elements of the first tape-head contact area are aligned with the second write and read elements of the second tape-head contact area so that an end portion of the second write element lies in a region that laterally overlaps only an end portion of the first write element. Data track widths less than one micron are achievable with this head design.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. Ser. No. 11/084,412, filedMar. 18, 2005, which is hereby incorporated by reference in its entiretyfor all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to storage devices, and moreparticularly to servo systems for compensating for lateral tape motion.

2. Related Art

Increased data storage capacity and retrieval performance are desired ofall commercially viable mass storage devices and media. In the case oflinear tape recording, a popular trend is toward multi-head,multi-channel head structures with narrowed recording gaps and datatrack widths, so that many linear data tracks may be laid on a tapemedium of a predetermined width, such as one-half inch width tape. Toincrease the storage density for a given cartridge size, the bits on thetape may be written to smaller areas and on a plurality of parallellongitudinal tracks. As more data tracks are recorded on a tape, eachtrack becomes increasingly narrow. The tape therefore becomes moresusceptible to errors caused from the tape shifting up or down (calledlateral tape motion or “LTM”) in a direction perpendicular to the tapetravel path as the tape passes by the magnetic head. LTM may be causedby many factors including tape slitting variations, tension variations,imperfections in the guiding mechanism, friction variations, andenvironmental factors such as heat and humidity. These factors affectLTM in various ways. Some may cause abrupt momentary jumps while othersmay cause a static shift. Generally, LTM is unpredictable andunrepeatable.

In multi-head, multi-channel magnetic tape storage systems, randomlateral tape motion is generally a limiting factor in achieving highertrack densities, and thus higher user data capacity. In order tomaintain proper alignment of the head with the storage tape and datatracks on the tape, the tape is generally mechanically constrained tominimize LTM and data retrieval errors. Mis-registration between thehead and data tracks can cause data errors during readback, and dataloss on adjacent tracks during writing.

Various techniques for increasing the track density on magnetic tapeemploy recording and maintaining servo information on the tape toprovide positioning information to a tape drive system during writingand/or reading processes. Some systems magnetically prerecord acontinuous track of servo information which is then read and used as aposition reference signal. For example, a variety of techniques havebeen used including dedicated, embedded magnetic servo tracks, time andamplitude magnetic servo tracks, and the like. One disadvantage ofprewritten servo tracks is that the use of a separate servo track writerintroduces errors into the system, reducing the ability to obtain thesmallest track width.

SUMMARY OF THE INVENTION

According to a head positioning servo method of an embodiment of theinvention, a head assembly includes a first head having first and secondwrite elements, and a read element. The method comprises the first writeelement writing a first data track, and the second write element writinga first reference track. The first reference track partially overwritesthe first data track to form a first servo edge for laterally servoingthe head assembly.

The second write element may write the first reference track while thefirst write element writes the first data track. The first referencetrack may comprise an erasure. Alternatively, the first data track maycomprise a first tone at a first frequency, and the reference track asecond tone at a second frequency. After moving the first head towardthe first servo edge, the read element may read information from thereference and first data tracks on both sides of the first servo edge toservo the head assembly.

The first head may be moved to a next data track position for writing.According to one embodiment, however, the first write element may notwrite while the head is servoing. If, on the other hand, the head is notservoing, then the first write element may write the next data track,which may partially overwrite the first data track to trim the firstdata track.

In an alternative embodiment, the head may write the next data trackwhile servoing off the servo edge from the previous track. In that case,the head temporarily inhibits writing of the next data track to createat least one servo gap and the read element servos off the first servoedge in the at least one servo gap to servo the head assembly.

The data track width may be based upon the lateral gap between the readelement and the second write element plus a portion of the lateral widthof the read element overlapping the first reference track when the headis properly positioned with respect to a next data track. Using theservo method of an embodiment of the invention, data track widths ofless than 6 microns, or even 1.0 micron, may be achieved.

According to another embodiment of the invention, a recording headcomprises a first tape-head contact area including a first write elementand a first read element, which itself may include an MR sensor element.The first read element is laterally offset as to the first writeelement, and no portion of the first read element lies in a regionlaterally overlapped by the first write element. A second tape-headcontact area includes a second write element and a second read element,which also may include an MR sensor element. The second read element islaterally offset as to the second write element, and no portion of thesecond read element lies in a region laterally overlapped by the secondwrite element.

The first read and write elements of the first tape-head contact areaare aligned with the second write and read elements of the secondtape-head contact area so that an end portion of the second writeelement lies in a region that laterally overlaps only an end portion ofthe first write element, the second read element lies in a region thatlongitudinally trails the first write element as to a first direction inwhich the first write element writes a first data track, and that islaterally overlapped fully by the first write element, and the firstread element lies in a region that longitudinally trails the secondwrite element as to a second direction opposite the first direction, andthat is laterally overlapped fully by the second write element.

The second write element is operable to write a first reference trackthat partially overwrites the first data track to create a referenceedge while the first write element writes the first data track. Thelateral offset between the second read and write elements is based upona width of the first data track less a portion of the lateral width ofthe second read element overlapping the first reference track when thehead is properly positioned with respect to a next data track. The headis operable to write the first data track on magnetic tape so that thefirst data track width is less than 1.0 micron, and may be even lessthan 0.6 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system according to an embodiment of the presentinvention.

FIG. 2 illustrates recording heads according to an embodiment of thepresent invention.

FIG. 3 is a schematic diagram illustrating an MR read/write setaccording to an embodiment of the present invention.

FIG. 4 illustrates a view of a tape bearing surface of the MR read/writeset of FIG. 3.

FIG. 5A illustrates the manufacture of the MR read/write set of FIG. 4according to an embodiment of the present invention.

FIG. 5B illustrates the manufacture of an alternative embodiment of theMR read/write set of FIG. 4 according to an embodiment of the presentinvention.

FIGS. 6-8 illustrate the writing of multiple tracks according to acontinuous servo embodiment of the present invention.

FIGS. 9A and B illustrate sampled servoing during a gap between datablocks according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable a person of ordinaryskill in the art to make and use the invention. Descriptions of specificdevices, techniques, and applications are provided only as examples.Various modifications to the examples described herein will be readilyapparent to those of ordinary skill in the art, and the generalprinciples defined herein may be applied to other examples andapplications without departing from the spirit and scope of theinvention. Thus, the present invention is not intended to be limited tothe examples described herein and shown, but is to be accorded the scopeconsistent with the claims.

FIG. 1 illustrates a servo system according to an embodiment of thepresent invention, which may be implemented in a media drive, such as atape drive, for example. The system includes a read/write head assembly100 according to an embodiment of the invention, read/write electronics102 for respectively reading and writing signals from and to the headassembly, a controller 104, and a positioning actuator 106 for laterallymoving the head assembly. Those skilled in the art will recognize thatthe controller may be implemented in firmware, software and/orassociated hardware. (Note that for purposes of this application, theterm “head assembly” denotes merely a ganged assembly of recordingheads, and does not necessarily include or exclude other components.)

Auto Servoing

Through the read/write electronics 102, the controller 104 reads thesignals output from the head assembly 100 reading a tape 108 located ina carrier, such as a cartridge or a cassette. Based upon the readsignals, the controller 104, executing a servo algorithm according to anembodiment of the invention, determines the position of the headassembly 100. The position may be represented by a position error signal(PES). The controller 104 uses the PES to instruct the actuator 106 toadjust the lateral position of the head assembly.

FIG. 2 illustrates an example of three recording heads ganged togetheras the first three heads of a head assembly according to an embodimentof the invention. Each head 200 includes complementary pairs of forwardand reverse read and write elements (transducers). In one embodiment,the head assembly includes 16 heads.

Here, assume that the forward direction of the tape denotes the tapemoving from left to right. In a linear tape drive, the tape typically iswritten its full length in one direction. Subsequently, the head moveslaterally to another track, the direction is reversed, and the tape isagain written its full length, creating a serpentine pattern.

According to one format, physical tracks may be grouped according tobands, channels and logical tracks. A physical track is the areatransversed on the tape medium by a recording head at a particularlateral position. Physical tracks laterally adjacent to each other andtraversed by the same recording head at different lateral positionswithin a band are identified as being associated with the same“channel.” (For the sake of convenience, we will also refer herein to ahead operating within a band as a “channel.”) For example, multiplephysical tracks written by one head in the same direction may belaterally adjacent to each other as a group, followed by another set ofphysical tracks associated with another channel. A head can continue tostep laterally to write physical tracks within a band, but stops beforeit reaches a track written by the next adjacent head in the headassembly, to avoid overwriting the track written by the next adjacenthead. The group of physical tracks associated with a channel correspondsto a “logical track.” Unless otherwise indicated herein, any referenceto “track” herein refers to a physical track. All the physical trackstraversed in one direction by all recording heads in a head assemblyrepresent a “band.”

When the tape is moving in the forward direction during a normal writeoperation, a forward read element (or “reader”)(e.g., RF1) trails behinda forward write element (or “writer”)(e.g., WF1) to verify the datawritten by the forward writer. Conversely, when the tape moves in thereverse direction, a reverse reader (e.g., RR1) trails behind thereverse writer (e.g., WR1) to verify the data.

Assume that the tracks are numbered in descending order from top tobottom according to the embodiment of FIG. 2, so that the first track(track 0) written by a particular head lies above the next track (track1). According to this frame of reference, the forward reader shown inthe figure lies laterally directly above the reverse writer. Thoseskilled in the art will recognize that the forward reader may lieanywhere in a region laterally above the reverse writer. More generally,those skilled in the art will recognize that the forward reader lies ina region laterally trailing behind the reverse writer with respect tothe lateral direction in which the head travels to servo using the servoedge created by the intersection of the data track and the referencetrack. An end portion of the reverse writer lies in a region thatlaterally overlaps an end portion of the forward writer within the head.

The forward reader and the reverse writer together are formed duringmanufacture on the same substrate using known photolithographictechniques for manufacturing thin film recording heads.

FIG. 3 is a schematic diagram of an example of a magneto-resistive(“MR”) read/write set (“R/W set”) 300 according to an embodiment of theinvention. The R/W set 300 includes a bottom shield 302, a top pole 304,and a shared write pole/shield 306 made of magnetic material. Aconnecting arm magnetic circuit 308 is integrally formed with andconnects the shared shield 306 and the top pole 304. The top pole 304,connecting arm 308 and shared shield 306 together form a write element.A write coil 310 is also formed using commonly known thin-film headmanufacturing technology.

An insulator is formed between the shared shield and the bottom shieldto form a first read gap 312. An MR read element 314 is located in thefirst read gap 312. Electrical contact is made to the MR read element314 through two leads 316.

FIG. 4 illustrates a view of the tape bearing surface of the R/W set ofFIG. 3. The R/W set has been reoriented in the figure so that the tapeis moving in a vertical direction on the page. According to anembodiment of the invention, the MR read element 314 is laterally offsetas to the top pole 304 of the write element, and no portion of the readelement lies in a region laterally overlapped by the top pole of thewrite element.

The write element is laterally wider than the read element. Note thatthe width of the write element is the same as that of the top pole. Theshared shield 306 is at least as wide as the sum of the lateral widthsof the MR read element 314, the write element 304 and the lateral offsetbetween the MR read element 314 and the write element 304, in order toreduce stray magnetic fields. The bottom shield 302 is at least as wideas the shared shield to reduce stray magnetic fields, as well.

The R/W set of the embodiment shown in FIGS. 3 and 4 may be formed usingstandard photolithographic techniques for manufacturing thin film MRheads. FIG. 5A illustrates the process of manufacturing the R/W set ofFIG. 4. As is known in the art, a R/W set may be typically formed on orwithin a tape-to-head (or “tape-head”) contact area, such as a bump orisland, where the tape to head contact is accomplished. Although theexamples herein refer to “bumps,” those skilled in the art willrecognize that the invention applies to a tape-head contact area,whether or not raised or indented, or actually in physical contact withthe tape medium.

The R/W set may be formed on a ALTC substrate or other appropriatematerial “puck” 500, upon which is deposited a base layer 502. Thebottom shield 302 is deposited on the base layer 502 over the substrate500. An insulator is deposited on the bottom shield to form a first readgap 504. The MR read element 314 is formed on the first read gap 504using standard photolithographic/masking techniques.

As an example, the MR read element 314 includes an MR sensor (such asNiFe or similar material 508), an insulative spacer 506, and a referencemagnetic material 510, such as SAL. Permanent magnets 512 are formed onboth sides of the MR read element 314. Conductors 514 are formed overthe permanent magnets 512 to provide the leads for sensing electricalchanges in the MR read element as it passes over magnetic tape. Theeffective width of the MR read element is determined by the width of theMR sensor.

An insulator is deposited over the MR read element 314 to form a secondread gap 516. The shared shield 306 is formed over the second read gap516, followed by a non-magnetic write gap 518. The top pole 304 isformed over the write gap 518 and the write coil and the connecting arm(not shown), and encapsulated using standard techniques.

Referring to FIG. 5A, the masks used to form the MR read element 314 arelaterally offset a predetermined distance from the center of the bottomshield to form an offset MR read element 314. The mask used to form thetop pole is centered over the shared shield. The top pole is wider thanthe MR read element 314.

Referring to FIG. 5B, according to another embodiment of the invention,the MR read element 314 is centered as to the bottom shield, and themask used to form the top pole is laterally offset from the MR readelement by a given distance. Those skilled in the art will recognizethat the read and write elements may be laterally placed in manydifferent regions to accomplish the desired offset.

At a predetermined distance away from the tape bearing surface, theconnecting arm is formed in the longitudinal direction between the toppole and the shared shield. The connecting arm passes through the writecoil. (The connecting arm and write coil are not shown in this figure.)

To form a head that can be used for read after write verification, asecond bump including a complementary R/W set is aligned with the firstbump on the substrate. As shown in FIG. 2, the first bump may be treatedas including the forward reader and the reverse writer, whereas thesecond bump includes the reverse reader and the forward writer. Thebumps are aligned so that an end portion of the reverse writer lies in aregion that laterally overlaps only an end portion of the forwardwriter, the forward reader lies in a region laterally overlapped fullyby the forward writer, and the reverse reader lies in a region laterallyoverlapped fully by the reverse writer.

Forming the reverse (forward) writer used to form the servo edge on thesame bump as the forward (reverse) reader used to read the servo edgeallows a very high precision in the gap between the “lateral” read/writeelement pairs. According to one embodiment, each write element may be 10microns long, and each read element 4 microns. The overlap between theend portions of the write elements may be 1.5 microns, and the gapbetween the read and write elements on the same island may be 3.5microns. As will be seen below, this precision translates to preciseplacement of extraordinarily narrow tracks according to the invention.

The auto-servo system of an embodiment of the invention allows eithercontinuous, dedicated servoing or sampled servoing. One or more tracksmay be dedicated for continuous servoing. In an example describedherein, adjacent channels 0 and 1 are alternately used for continuousservoing. Those skilled in the art will recognize, however, that headsin any location, such as at opposite ends of the head assembly, may beused for continuous servoing.

FIGS. 6-8 illustrate the read and write functions performed by a headimplementing an example of an auto-servo algorithm according to acontinuous servo embodiment of the invention. Referring to FIG. 6 andfocusing on the first head (i.e., channel 0) for now, during a firstpass the forward writer W_(F) writes data to a data track (track 0). Thefirst pass may be written open loop. Alternatively, the lateral positionof the head assembly during the first pass may be controlled by servoingoff the edge of the tape with an optical mask or by using othertechniques, such as those described in “MASKED POSITION SENSORS ANDCONTROL SYSTEMS,” application Ser. No. 10/942,678, filed Sep. 15, 2004(the “'678 application”), and “DIFFRACTIVE POSITION SENSORS AND CONTROLSYSTEMS,” application Ser. No. 10/927,732, filed Aug. 27, 2004 (the“'732 application”), assigned to the assignee of the present invention,and incorporated by reference in their entirety herein.

In conventional tape drives, the reverse writer is not used duringforward writing. According to an embodiment of the invention, however,the reverse writer W_(R) writes a reference track during the forwardpass. The reverse writer may, for example, write a simple DC erasesignal or a signal at a preset frequency, such as 0-10 KHz. As a result,the reverse writer trims the first data track. As will be seen below,the intersection between the reference track and the data track forms areference edge (otherwise denoted herein as a “servo edge”) 600 used tolaterally position the head assembly during subsequent passes.

Using coarse positioning as is known in the art (or as described in the'678 or '732 applications), for the next pass the controller causes theactuator to step the head assembly laterally to the next track position(e.g., track 1) within a given coarse tolerance. Referring to FIG. 7,this places the forward reader RF of channel 0 in the vicinity of theservo edge. The system may be calibrated so that a predetermined portionof the forward reader reads from the reference track (and acomplementary portion reads from the data track) when the head assemblyis properly positioned with respect to the data tracks.

For example, the forward reader may be calibrated so that it reads 50%from the reference track and 50% from the data track when properlypositioned. During the first pass when the forward writer writes data,the forward reader fully overlaps the first data track. The resultingsignal can be used as a calibration reference indicating 100% overlap.During the next (current) pass, if the reference track was created usinga DC erase signal, then the amplitude of the read signal may becorrelated directly with the degree of overlap. Thus, 50% overlap may beindicated by reading a signal that is 50% of the full amplitudeassociated with a 100% overlap. The degree of overlap indicates thelateral position of the forward reader, and thus the position of theentire head assembly.

As an alternative, the servo may use a dual frequency technique. In thiscase, the forward writer writes to the data track a first tone of apredetermined first frequency along with the data. The first tone isselected so as not to interfere with the data. For continuous servoing,the first tone may be written throughout the data over the entire track,whereas for sampled servoing, the first tone need only be written in thetape locations that are sampled for servoing. The reverse writer writesa second tone of a predetermined second frequency to the referencetrack. The second frequency is selected to be low enough (e.g., 0-10KHz) so as not to couple into the forward reader on the same island.

The forward reader reads both tones when overlapping the servo edge. Thecontroller performs signal processing (e.g., filtering) to isolate theread tones, and determines the difference in their amplitudes todetermine the forward reader position. Using the position measured (byeither technique), the controller instructs the actuator to adjust thelateral position of the head assembly to the correct location.

If a particular head (e.g., channel 2) is not in servo mode, then it maybe used to write the next data track (track 1) (upon which it iscurrently positioned in FIG. 7). When this happens, the forward writerof channel 2 partially overwrites the previous data track (track 0) totrim the width of the previous track (to, e.g., 5.5 microns). As part ofthis process, the next data track completely overwrites the servo edgewritten during the previous track. From the writing of a track and thenext adjacent track, one can see that the resulting track width is equalto the lateral gap between the forward read element and the reversewrite element plus a portion of the lateral length of the forward readelement that would overlap the corresponding reference track if the headwere properly positioned with respect to the next data track. Pleasenote that, in the claims, references to the width of a data track are tothe width of a data track not used for servoing (in the dedicated servomode), e.g., channel 2 in this example, that results after writing thenext adjacent track.

Holding other factors constant, to implement the auto-servo algorithm toachieve a given track width, the lateral offset between the read andwrite elements may be chosen as the ultimate data track width (aftertrimming by both the reverse writer and the forward writer writing thenext data track) less the portion of the lateral width of the forwardread element overlapping the reference track when the head is properlypositioned with respect to the next data track. Viewed another way, ifother factors are predetermined, then the lateral width of the forwardwriter may be chosen as the sum of the ultimate data track width, thelateral offset between the forward reader and the reverse writer, andthe portion of the lateral width of the forward reader overlapping thereference track when the head is properly positioned with respect to thenext data track.

Using this technique and current head manufacturing capabilities, a headaccording to an embodiment of the invention can write trimmed datatracks as narrow as 1.0 micron, or less, such as 0.55 micron. Thecurrent state of the art in thin film head manufacturing, in combinationwith the auto-servo algorithm of the invention, can support, forexample, a 0.55 micron track width using the following dimensions:forward (and reverse) write element length=1.0 micron; forward readelement length=0.4 micron; gap between reverse write element and forwardread element=0.35 micron; and percentage of the lateral length of theforward read element that would overlap the corresponding referencetrack if the head were properly positioned with respect to the next datatrack=50%.

Referring to FIG. 7, the channel 0 head described above (denoted the“first head” for the sake of convenience) may be used for servoing offof its corresponding servo edge during the entire pass of the headassembly in one direction. Referring to FIG. 8, on the next pass in thesame direction, another head (denoted the “second head” for the sake ofconvenience) located anywhere else in the head assembly (channel 1 inthis example) may servo off its corresponding servo edge. (It isimplicit that the tape is rewound between passes in the same direction.)

By having the channel 0 head freed from servoing during this pass, itmay write data to track 2. During the next pass (not shown), the channel0 head servos off the servo edge 800 created during the previous pass(shown in FIG. 5), while the channel 1 head is free to write data totrack 3 (not shown). Note that, to create consecutive servo edges, thetwo servoing channels in this example behave differently than the otherchannels. Specifically, to prevent overwriting of their servo edges,channel 0 does not write odd-numbered tracks, and channel 1 does notwrite even-numbered tracks. Channels 2-15 (in a 16-head head assembly)write both odd and even tracks.

During continuous servoing, when the controller employs the forwardreader of a head (e.g., channel 0) for servoing, the head that includesthe servoing reader does not write data. During a normal data writeoperation in the forward direction, the forward reader is used to verifythe data written by the forward writer. During continuous servoing, theforward reader cannot perform this function. Nevertheless, other headsin the head assembly (e.g., channels 1 and 2) can perform normalread-verified write operations while the head that employs its readerfor servoing helps servo the entire head assembly.

FIGS. 9A and B illustrate an example of two passes of a recording headpositioned for sampled servoing during a gap between data blocks. (Inthe figure, the head width relative to the gap width is exaggerated forillustrative purposes.) In actuality, the servo gap is much larger thanthe read element longitudinal width. Those skilled in the art will alsorecognize that, although the figure only illustrates one servo gap forone channel, in practice the algorithm of an embodiment of the inventionwould likely employ many servo gaps dispersed over the tape length.Moreover, more than one channel may include servo gaps for sampledservoing. As with the continuous servo algorithm, heads in any location,such at opposite ends of the head assembly, may be used for sampledservoing.

Using the sampled servo method, the data is written in blocks by theforward writer, separated by gaps. According to an embodiment of theinvention, the controller inhibits the forward writer W_(F) (e.g., ofthe head associated with channel 1 in this example) from writing (e.g.,track 1) during a predetermined time period, as shown in FIG. 9B. Theresulting “servo” gap 900 includes the data previously written by theforward writer during the previous pass (shown in FIG. 9A), whilewriting track 0, which was trimmed by the reverse writer W_(R) to createa servo edge 902. Using the dual frequency technique, during the writingof track 0 the forward writer wrote a first tone and the reverse writerwrote a second tone, which is also exposed in the gap associated withtrack 1. Thus, the forward reader may servo off the servo edge 902exposed in the gap using the dual frequency method. Of course, thesampled servo technique may also be used in the case that the forwardwriter only writes regular data and the reverse writer creates an erasedreference track.

When the forward reader reaches the servo gap, the controller uses thereader for servoing. At that time, unlike the continuous servo method,the forward writer can write data, and the forward reader can verify thedata after it exits the gap and reaches the newly written data. Theforward writers for the other heads in the assembly may also be activeto write data when a head is servoing. Alternatively, more than onehead, or even all heads, in the head assembly may be employed forservoing during the gaps occurring in their respective tracks. (Pleasenote that the gap used for servoing in one track is typically notnecessarily longitudinally aligned with the servo gaps in other tracks.)

Another advantage of the sampled servo technique over continuousservoing is that, instead of alternating heads, the same head may beused for servoing at all times. Because no two heads can be perfectlymatched in structure, positioning and impedance, among other factors,the differences in the characteristics of the alternating headsintroduces some error into the continuous servoing process and thelimitation of the number of dedicated heads redundancy, on the otherhand, the sampled servo method avoids any such differential error. Thecontinuous servo technique also detracts from data capacity because ofthe use of dedicated heads.

During sampled servoing, the controller should avoid conventional,non-servo gaps that result from the usual recording of data blocks ontotape. These non-servo gaps will not include the servo edge used forservoing by the invention. Also, the servoing head must avoid servoingoff of servo gaps created during a previous pass on an adjacent track.We will refer to both types of gaps as “invalid gaps.” Using knowndesynchronization techniques (e.g., by controlling timing and/or spatialfrequency of the gaps), the controller of an embodiment of the inventioncontrols the placement of the (valid) servo gaps to avoid these invalidgaps.

By writing the servo edge during normal write operations, the presentinvention avoids the use of a separate servo writer that would introduceerrors into the servo process. Further, by using the same heads to writethe servo edge as regular data, the invention allows for a closecorrelation between the servo edge and the actual data tracks. Moreover,by trimming the tracks using the reverse writer, which is on the samesubstrate as the forward reader, and by using the forward writer itselfwhen writing the next track, the present invention allows for preciseplacement of very narrow tracks.

One skilled in the art will recognize that the most recently writtenservo edge may be used for servoing during a read operation by any headin the assembly. In other words, while the forward reader servos off itscorresponding servo edge to servo the head assembly, a forward reader ofanother head in the assembly may perform a normal read operation.

Write Position Verification

The recording head of an embodiment of the invention may be employed notjust for servoing, but for other functions, as well. As noted above forthe sampled servo write algorithm, during the servo gap the read elementmay read the servo edge of the previous track. Instead of using the headfor servoing, however, the head may be used to accurately verify itsposition where it is servoed using other techniques, e.g., magnetic oroptical.

Using conventional servoing, lateral head position is typicallydetermined by a surrogate for the head, e.g., the location of opticaltracks on the back of the tape. The servo edge created by the head ofthe invention, however, provides a direct reference to the actual headposition. In some instances, a conventional head servoed by traditionalmeans may wander off track due to, for example, an error in the opticalservo tracks on the back of the tape, or a servo writing error. This mayhappen in conjunction with successful read after write verificationbecause the reader does not track head position. As a result, aconventionally servoed writer may overwrite existing tracks.

Using the head of the invention, the head creates a reference edge, asbefore, during the writing of a data track. During the writing of a nextadjacent track, the head inhibits writing during predetermined periodsto create a “reference gap.” Within the reference gap the head mayattempt to read the reference edge. The controller will know what signaltypes and quality to expect while the read element is within the gap.For example, the head may attempt to read the first and second tonesusing the dual frequency technique. If, however, the controller does notreceive the expected signal at a predetermined threshold quality, thenthat condition may indicate that the head is off course by anunacceptable amount, possibly leading to the danger of overwritingadjacent tracks. In that case, the controller may inhibit writing of thenext data block. Presumably, the servo mechanism, e.g., the opticalservo, would correct course after a predetermined delay, e.g., by thetime the data block after the skipped block is to be written. Thus, thecontroller may instruct the head to resume writing after the delay.

It will be appreciated that the above description for clarity hasdescribed embodiments of the invention with reference to differentfunctional units. However, it will be apparent that any suitabledistribution of functionality between different functional units may beused without detracting from the invention. Hence, references tospecific functional units are only to be seen as references to suitablemeans for providing the described functionality rather than indicativeof a strict logical or physical structure or organization.

The invention can be implemented in any suitable form includinghardware, software, firmware or any combination of these. Differentaspects of the invention may be implemented at least partly as computersoftware or firmware running on one or more data processors and/ordigital signal processors. The elements and components of an embodimentof the invention may be physically, functionally and logicallyimplemented in any suitable way. Indeed the functionality may beimplemented in a single unit, in a plurality of units or as part ofother functional units. As such, the invention may be implemented in asingle unit or may be physically and functionally distributed betweendifferent units and processors.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Rather, the scope of the present invention is limitedonly by the accompanying claims. Additionally, although a feature mayappear to be described in connection with a particular embodiment, oneskilled in the art would recognize that various features of thedescribed embodiments may be combined in accordance with the invention.

Moreover, it will be appreciated that various modifications andalterations may be made by those skilled in the art without departingfrom the spirit and scope of the invention. For example, those skilledin the art will recognize that, although the invention has beendescribed in the context of tape moving in the forward direction, theinvention is equally applicable (using the complementary set of read andwrite elements) to tape moving in the reverse direction. If the samehead configuration as that described herein is used in the reversedirection, then the algorithm of the invention would write tracks in alateral direction opposite that described herein, e.g., track 1 would beabove, not below, track 0. In general, while servo writing, the headmust move laterally in the direction of the newly created servo edge.Moreover, although the servo control algorithm of an embodiment of theinvention was described using a particular head configuration, thoseskilled in the art will recognize that the algorithm may be practicedwith other head configurations. Further, although the invention hasgenerally been described in the context of a linear tape drive, it isnot to be so limited. The invention is not to be limited by theforegoing illustrative details, but is to be defined according to theclaims.

1. A head positioning servo method for non-azimuthal recording, whereina head assembly includes a first head having first and second writeelements and a read element, the method comprising: writing a first datatrack with the first write element; and writing a first reference trackwith the second write element, wherein the first reference track iswritten while the first write element writes the first data track andthe first reference track partially overwrites the first data track toform a first servo edge for laterally servoing the head assembly,wherein the head assembly further includes a second head includingcorresponding first and second write elements and a read element, theread element of the second head for servoing off a second servo edgeformed by data and reference tracks respectively written by the firstand second write elements of the second head; and the read element ofthe second head servoing off of the second servo edge to laterallyposition the head assembly while the first head writes the first datatrack.
 2. The method of claim 1, wherein the first head alternates withthe second head to servo off of its corresponding servo edge while theother head writes data to a corresponding data track.